Mixers are important building blocks in radio technology, and are designed to multiply two or more input AC signals with each other, and to output one or more AC products of the input signals, relying on the “addition and subtraction” which arises when two AC signals are multiplied by each other.
Due to the “addition and subtraction” mentioned above, mixers can be used both to “up-convert” and to “down-convert” signals, i.e. a signal at a certain frequency can by means of multiplication by another signal in a mixer be transposed to a higher or a lower frequency.
In a radio application, for example, a mixer can be used to down-convert a high frequency RF signal, fRF, into a low frequency IF signal, fIF, or vice versa, by means of multiplication with a signal from a Local Oscillator, an LO. In down-conversion, the IF frequency fIF is equal to the difference of the RF frequency, fRF, and the LO frequency, fLO, so that fIF=fRF−fLO.
A special group of mixers are so called sub-harmonic mixers, which utilize harmonics of the LO signal, i.e. N*fLO, where N is an integer.
With increasing RF frequencies, for instance in applications on the E-band, sub-harmonic mixers may be more attractive, since they require an LO frequency which is half of that of a conventional mixer, i.e. 2*fLO if N=2. In such a case, in the case of down-conversion, fIF=fRF−2*fLO, since the obtained IF frequency is a mixing product of the RF signal and the second harmonic of the LO signal. Another advantage of sub-harmonic mixers is that an LO source which operates at a low frequency has a low phase noise.
However, sub-harmonic mixers usually exhibit low conversion gain, high LO power consumption, and poor linearity.
Another factor which is often looked at in sub-harmonic mixers is their linearity, which is often measured by the so called IMD3, i.e. third order intermodulation products in two-tone measurements. A lower IMD3 indicates a better linearity.